The present invention generally relates to an art for preventing damages of printed matters due to static electricity, and particularly to a printing method for preventing various damages due to static electricity in a stack of plural paper sheets subjected to electrostatic printing.
As is well known in the art, in rotogravure printing, recess portions (cells) of dots representing characters or figures provided on the peripheral surface of a gravure plate cylinder are filled with ink. Then, continuous printing paper is allowed to pass the peripheral surface of the gravure plate cylinder while being pressed against the same by means of an impression cylinder, so that the ink in the cells transfers to a contact surface of the printing paper, thereby causing thecharacters or figures to develop on the paper surface.
In order to facilitate transfer of the ink from the inside of the cells to the printing paper, static electricity is used. Thus, in electrostatic gravure printing process, while continuous paper fed from a roll of paper is passed between the gravure plate cylinder and the impression cylinder, the paper is pressed against the surface of the plate cylinder by the impression cylinder, so that the printing ink filled in the cells of the plate surface of the gravure plate cylinder transfers to the surface of the paper, and consequently printing is accomplished.
The gravure printing ink is composed of electrically neutral fine particles. In the electrostatic printing, in order to achieve effective transfer of the ink from the cells of the plate cylinder to the surface of the paper, an electric field is generated in a nip portion between the plate cylinder and the impression cylinder. Thus, it is easy for the particles of the printing ink in the cells of the plate cylinder passing through this electric field to transfer from the cells to the surface of the paper by employing the force generating in the electrostatic field. Since the paper and the ink pass through the electrostatic field, the printing surface of the paper subjected to the electrostatic printing is uniformly charged positively and negatively.
In an automatic production process of magazines, catalogues and the like, continuous paper is printed at both sides and cut and folded into signatures. Then a certain number of signatures for forming a single book are stacked in the order of page, pushed at both sides thereof, tightly bound as one body, and fed into a book binding process, and consequently the stacks are formed into books.
The book produced in the above manner sometimes causes a problem that when a reader tries to turn pages, he can not easily separate each pair of pages opposing with each other because the opposing pages adhere to each other by electrostatic attraction. In such a case, if the reader misses the adhered pages or skips the pages without an effort of peeling the adhered pages, there remain pages that are not viewed by the reader, which will cause a trouble especially in the case of a sales catalogue. In addition, crackling sounds occurring at the time of turning pages will make the reader unpleasant. It is considered that the unpleasantness is caused by so-called electric discharge induced by separation.
In the book produced in the above manner, positive and negative electric charges due to the electrostatic printing remain uniformly on a printing surface of each page. Then, it is considered that between opposing pages of the book, an electrostatic force of f=kq1q2/r2 acts according to Coulomb""s law. Therefore, as the remaining electric charges increase, the effect of the electrostatic force increases. Furthermore, it is considered that the opposing pages of the book locally form a single capacitor consisting of oppositely charged printing surfaces with an air gap interposed therebetween. As is well known in the art, when dielectric constant of the air is defined as xe2x80x9cxcex5xe2x80x9d, distance between the opposing pages as xe2x80x9cdxe2x80x9d and opposing surface area as xe2x80x9cSxe2x80x9d, the capacity C of this capacitor is represented by the formula:                     C        =                              ϵ            ⁢                          xe2x80x83                        ⁢            S                    d                                    (        1        )            
And when electric charge of the capacitor is defined as Q. electricpotential difference between the opposing pages, from the well-known formula Q=CV(2), V is represented as:
V=Q/C xe2x80x83xe2x80x83(3)
As described above, since each page of the book is pressed and tightly bound in the course of bookbinding, xe2x80x9cdxe2x80x9d in the above formula (1) decreases to cause xe2x80x9cCxe2x80x9d to increase, and therefore, xe2x80x9cVxe2x80x9d decreases with increase in xe2x80x9cCxe2x80x9d because xe2x80x9cQxe2x80x9d is fixed in accordance with a voltage applied in printing. However, when a readers turns the pages of a book, opposing pages are opened to rapidly increase xe2x80x9cdxe2x80x9d and hence xe2x80x9cCxe2x80x9d rapidly increases in the formula (1). As a result of this, the voltage difference V between the opposing pages forming the capacitor rapidly increases according to the formula (3) so that the electric charges cause so-called peeling electric discharge between these pages. This would be the reason for generation of the above-mentioned unpleasant sounds.
Paper conventionally used for gravure printing has a surface resistivity of generally more than or equal to 1010xcexa9/xe2x96xa1. However, if electrostatic printing is performed by using this paper, the electrostatic troubles as mentioned above will occur due to the electric charges. As the surface resistivity of the paper is decreased to reduce the electric charge and rapidly attenuate static electricity, the surface conductivity of the paper is increased and the electric field strength of the aforementioned nip portion is weakened. Consequently, it is likely to generate so-called xe2x80x9cmissing dotxe2x80x9d on the printing surface and deteriorate the quality of the printed matter. In recent merchandise catalogues, the number of pages tends to increase, and the desire to use thin paper arises from the viewpoint of transportation costs. However, such thin paper is much affected by the electric charges in turning of a page of printed matters because the firmness of the paper is weak.
The present invention suppresses occurrence of missing dots on the printing surface and prevents the above-mentioned electrostatic troubles from occurring due to the remaining electric charges in electrostatic printing by using paper having a surface resistivity of a predetermined range in electrostatic gravure printing.
According to a method of the present invention, paper having a surface resistivity in the range of 1.0xc3x97109 to 9.0xc3x97109 xcexa9/xe2x96xa1, preferably 1.0xc3x97109 to 7.0xc3x97109 xcexa9/xe2x96xa1, most preferably 1.0xc3x97109 to 5.0xc3x97109 xcexa9/xe2x96xa1 is used. These values of surface resistivity are measured under the condition that a water content of the paper is in the range of 4% to 6%. The water content is measured under the environmental condition of 23xc2x0 C.xc2x11xc2x0 C. in temperature and 50%xc2x11xc2x0 C. in temperature and 50%xc2x12% in RH (relative humidity).